Police on Wednesday fired tear gas to disperse hundreds of Salafist demonstrators outside the US embassy in Tunis gathered to condemn a film deemed offensive to Islam, an AFP photographer said.

He said the intervention came as the demonstrators, estimated to number 300, started to try to break through the gates of the embassy compound after having demonstrated peacefully for several hours outside.

Earlier on Wednesday, Tunisian President Moncef Marzouki denounced the killing of US Ambassador Chris Stevens and three of his American co-workers in the eastern Libyan city of Benghazi as a "terrorist act."

"These dirty acts of terrorism ... are also a terrible blow to Tunisia," he said in broadcast remarks. "The security of Libya is also the security of Tunisia."